The Tonight Show is the crown jewels of American television. It has been called the most important slot in showbusiness, and a monument of US culture.

On Friday, after 17 years, its host will sit at the famous studio desk for the final time. Jay Leno is vacating the post he has held since 25 May 1992.

His final guest is the man to whom he is passing the baton, the strawberry-blond and elongated comic Conan O'Brien.

Their encounter will be a TV moment: the switch from Leno, 59, to O'Brien, 46, is a jump in generations. It is also a move away from Leno's ­gentler, more home-style comedy of the ordinary guy next door to the spikier humour of his successor. NBC has wanted to make the change for many years, ­actually laying down the terms of the transition as long ago as 2004.

The network sees O'Brien as a means of revitalising the late-night talkshow ­format that America embraced with the first Tonight Show in 1954.

Leno has proved hugely popular, with 5 million viewers, but their profile is ­ageing. O'Brien on the other hand has more ­purchase with young people, particularly males, who are attractive to advertisers.

The end of The Tonight Show with Jay Leno also brings to a close the head-to-head rivalry with David Letterman, a ­running theme over 17 years. The two were fierce contenders to take over from the programme's demigod, Johnny ­Carson, who ruled for 30 years.

Carson, it is presumed, wanted to hand over to Letterman, but Leno stole the slot.

Leno got off to a rocky start, trailing ­Letterman, who had by now taken the same 11.30pm slot on CBS.

It was not until Hugh Grant came to the rescue, making an infamous appearance shortly after his arrest in Los ­Angeles in 1995 for employing the services of a ­prostitute, that Leno pulled ahead of his rival in the ratings.

He has been in front ever since. However, his brand of chummy wisecracks and his soft-ball treatment of his guests has consistently failed to find favour with critics, who have tended to prefer the acerbic wit and character assassination of Letterman.

Leno's soft demeanour and cherub-faced looks give the impression of ­affability. But he is quite capable of playing hardball, as his bosses at NBC have discovered.

To begin with he respected their arguments for wrenching him from the chair, explaining to his audience on the 50th anniversary of the Tonight Show: "You can do these things until they carry you out on a stretcher; or you can get out when you're still doing good."

But he grew progressively more disgruntled and bloody-minded about the change. NBC executives began to fret that Leno was preparing to defect to another network, taking his devoted following of ageing middle Americans with him.

After a prolonged bout of shadow boxing, it was agreed that he would be allowed to continue with his own show at the ­earlier hour of 10pm. The Jay Leno Show will begin in September, five nights a week.

It will be another make-or-break moment for the jokemeister. Unlike the Tonight Show, whose competitors are fellow comics – Letterman and Stephen Colbert – he will now be up against blockbuster crime series such as CSI: Miami.

It will either be a triumph, proving that the talkshow, which has the advantage of being considerably cheaper to produce, can take on blood-soaked dramas and win; or it will be his undoing.

Leno appears to be relaxed either way. He is not a one-track TV personality. He has a collection of more than 100 vintage cars, his private passion.

He also continues with striking humility to crisscross the country, performing as a stand-up artist. His next show is on Saturday in casino-rich Atlantic City in New Jersey.

He will be appearing there live on stage, no cameras present, hours after he bids his final farewell to the temple of American TV comedy.